1. Field of the Invention
The present invention generally relates to medical methods and apparatus. More particularly, the present invention relates to methods and apparatus used to restrict flexion of a fused spinal segment. The methods and apparatus disclosed herein may be used alone or in combination with fusion or other orthopedic procedures intended to treat patients with spinal disorders such as back pain.
A host of spinal conditions exist which often result in instability issues and/or back pain. A major source of chronic low back pain is discogenic pain, also known as internal disc disruption. Discogenic pain can be quite disabling, and for some patients, can dramatically affect their ability to work and otherwise enjoy their lives. Patients suffering from discogenic pain tend to be young, otherwise healthy individuals who present with pain localized to the back. Discogenic pain usually occurs at the discs located at the L4-L5 or L5-S1 junctions of the spine. Pain tends to be exacerbated when patients put their lumbar spines into flexion (i.e. by sitting or bending forward) and relieved when they put their lumbar spines into extension (i.e. by standing or arching backwards). Flexion and extension are known to change the mechanical loading pattern of a lumbar segment. When the segment is in extension, the axial loads borne by the segment are shared by the disc and facet joints (approximately 30% of the load is borne by the facet joints). In flexion, the segmental load is borne almost entirely by the disc. Furthermore, the nucleus shifts posteriorly, changing the loads on the posterior portion of the annulus (which is innervated), likely causing its fibers to be subject to tension and shear forces. Segmental flexion, then, increases both the loads borne by the disc and causes them to be borne in a more painful way. It would therefore be desirable to provide methods and apparatus that can be used alone or in combination with other spinal treatments to reduce loading in the area of the disc and adjacent tissue.
A number of treatments exist for addressing back pain and spinal instability. Some of these include, but are not limited to, fusion of the affected spinal segment. The patient may also be required to wear an external back brace for three to six months in order to allow the fusion to heal. Unfortunately, external braces are not always desirable since such braces can be uncomfortable, expensive, and inconvenient to use, and patient compliance often is low. An alternative to the back brace is to instrument the spinal segment with traditional instrumentation. Traditional instrumentation also facilitates fusion and prevents subsequent motion along the fused segment. While this treatment may be effective, it can also have shortcomings. For example, the fusion procedure with traditional instrumentation is more invasive, and when rigid instrumentation is used (e.g. pedicle screws and spinal stabilization rods), the instrumented region of the spinal segment becomes very stiff, and motion is prevented across the fusing segment. Loads can be borne by the instrumentation rather than the tissue, and loads and motion at adjacent segments can be increased. This is not always desirable, since a certain amount of motion and loading may actually help the healing process, promote fusion, and prevent excessive wear and tear on adjacent implants and tissue. Also, loading on the instrumentation may result in loosening or other mechanical failure of the instrumentation. Therefore, it would be desirable to have an improved device for instrumenting a fused spinal segment. It would also be desirable if an improved device minimized loads at the device/bone interface to minimize the potential of loosening and other mechanical failure. It would also be desirable if the device diminished the peak loading patterns at the bone/implant interface.
For the aforementioned reasons, it would therefore be advantageous to provide methods and apparatus that can be used with spinal fusion to help facilitate fusion of the vertebrae while still allowing some motion and loading of the fusion graft. It would be further desirable to provide methods and apparatus that are minimally invasive to the patient, cost effective and easy to use.
2. Description of the Background Art
Patents and published applications of interest include: U.S. Pat. Nos. 3,648,691; 4,643,178; 4,743,260; 4,966,600; 5,011,494; 5,092,866; 5,116,340; 5,180,393; 5,282,863; 5,395,374; 5,415,658; 5,415,661; 5,449,361; 5,456,722; 5,462,542; 5,496,318; 5,540,698; 5,562,737; 5,609,634; 5,628,756; 5,645,599; 5,725,582; 5,902,305; Re. 36,221; 5,928,232; 5,935,133; 5,964,769; 5,989,256; 6,053,921; 6,248,106; 6,312,431; 6,364,883; 6,378,289; 6,391,030; 6,468,309; 6,436,099; 6,451,019; 6,582,433; 6,605,091; 6,626,944; 6,629,975; 6,652,527; 6,652,585; 6,656,185; 6,669,729; 6,682,533; 6,689,140; 6,712,819; 6,689,168; 6,695,852; 6,716,245; 6,761,720; 6,835,205; 7,029,475; 7,163,558; Published U.S. Patent Application Nos. US 2002/0151978; US 2004/0024458; US 2004/0106995; US 2004/0116927; US 2004/0117017; US 2004/0127989; US 2004/0172132; US 2004/0243239; US 2005/0033435; US 2005/0049708; 2005/0192581; 2005/0216017; US 2006/0069447; US 2006/0136060; US 2006/0240533; US 2007/0213829; US 2007/0233096; 2008/0009866; 2008/0108993; Published PCT Application Nos. WO 01/28442 A1; WO 02/03882 A2; WO 02/051326 A1; WO 02/071960 A1; WO 03/045262 A1; WO2004/052246 A1; WO 2004/073532 A1; WO2008/051806; WO2008/051423; WO2008/051801; WO2008/051802; and Published Foreign Application Nos. EP0322334 A1; and FR 2 681 525 A1. The mechanical properties of flexible constraints applied to spinal segments are described in Papp et al. (1997) Spine 22:151-155; Dickman et al. (1997) Spine 22:596-604; and Garner et al. (2002) Eur. Spine J. S186-S191; Al Baz et al. (1995) Spine 20, No. 11, 1241-1244; Heller, (1997) Arch. Orthopedic and Trauma Surgery, 117, No. 1-2:96-99; Leahy et al. (2000) Proc. Inst. Mech. Eng. Part H: J. Eng. Med. 214, No. 5: 489-495; Minns et al., (1997) Spine 22 No. 16:1819-1825; Miyasaka et al. (2000) Spine 25, No. 6: 732-737; Shepherd et al. (2000) Spine 25, No. 3: 319-323; Shepherd (2001) Medical Eng. Phys. 23, No. 2: 135-141; and Voydeville et al (1992) Orthop Traumatol 2:259-264.